For the purposes of this specification, the term “tungsten bronze structure” refers not to HxWO3 (tungsten bronze) or hexagonal tungsten bronze structure (HTB) that are known for their electrochromic phenomena, but to what is generally known as tetragonal tungsten bronze structure (TTB). Note that the term “tetragonal tungsten bronze structure” is the name given to a particular crystal structure and does not immediately mean that the referred crystal is always tetragonal. There are orthorhombic materials also.
Regarding tungsten bronze structure oxides, the settings of the unit cell differ between the tetragonal and orthorhombic systems. In this description, the settings of the tetragonal system are used to indicate the crystallographic planes, crystallographic orientations, and diffraction.
In this specification, the term “morphotropic phase boundary” refers not only to a boundary at which the crystal system changes depending on the composition, which is a general definition, but also to a boundary or region at which the space group changes depending on the composition.
In this specification, the term “Curie temperature” refers not only to a temperature beyond which a material loses its ferroelectricity, which is a general definition, but also to a temperature at which the maximum dielectric constant is observed when measured by varying the measurement temperature using a minute alternating electric field of a particular frequency.
In this specification, the term “mol %” refers to a percentage of the amount of a designated element relative to the total amount of the substances occupying the designated sites.
The majority of piezoelectric materials used in various piezoelectric devices are perovskite piezoelectric materials containing lead, represented by lead zirconate titanate. However, efforts have been made to replace the lead-containing piezoelectric materials with lead-free piezoelectric materials. This is because it has been pointed out that when piezoelectric devices containing lead are discarded and exposed to acid rain, the lead component in the piezoelectric materials elutes into earth, possibly impacting the ecosystem. Proposals regarding lead-free piezoelectric materials have thus been made.
Regarding the perovskite piezoelectric materials, utilization of a morphotropic phase boundary (simply referred to as “MPB” hereinafter) has been investigated to improve the piezoelectric properties. For example, a group led by Fu reports in NPL 1 that the difference in free energy between crystals is small and this free energy changes depending on the applied electric field at the MPB, i.e., the boundary of different crystal systems. As a result, field-induced phase transition can be developed by the electric field. When the orientation of the spontaneous polarization axis, which is the strain direction of crystals, is rotated following the phase transition, a large displacement occurs. It is believed that the rotation of the spontaneous polarization is one of the mechanisms that impart high piezoelectric properties to the perovskite piezoelectric materials at the MPB.
Regarding piezoelectric materials other than those having the perovskite structure, tungsten bronze structure piezoelectric materials are known to have MPB. For example, a group led by Oliver reports in NPL 2 that they found MPB between BaNb2O6 and PbNb2O6. Some of the lead-containing tungsten bronze structure piezoelectric materials have a spontaneous polarization axis in the a-b plane orthogonal (inclined by 90°) to the c axis. When a tungsten bronze structure piezoelectric material containing lead and having a spontaneous polarization axis in the a-b plane is dissolved into a tungsten bronze structure piezoelectric material having a spontaneous polarization in the c axis direction, a MPB at which the spontaneous polarization axis direction changes can be formed. Actually, the improvements on piezoelectric properties at the MPB have been confirmed. However, regarding lead-free tungsten bronze structure piezoelectric materials, only those having a spontaneous polarization axis extending in the c axis direction have been known irrespective of the crystal system. Thus, even if a MPB is formed, a giant displacement caused by rotation of the spontaneous polarization axis can rarely be used. Therefore, discovery of lead-free tungsten bronze structure piezoelectric materials having spontaneous polarization axes oriented in directions other than the c axis direction (also referred to as “non-c axis” hereinafter) is desirable to use the rotation of the spontaneous polarization axis to improve the piezoelectric properties.
A group led by Muehlberg discloses in NPL 3 a solid solution system of BaNb2O6 and CaNb2O6 as a lead-free tungsten bronze structure piezoelectric material. However, the piezoelectric properties of this material are significantly low.
PTL discloses a piezoelectric ceramic composition containing, as a main component, a tungsten bronze structure complex oxide that contains metal elements, Na, Ba, Bi, and Nb, in which the ratio of Bi to the total weight is 3 to 6 wt % on a metal basis and the Na content is significantly larger than the Bi content. This piezoelectric ceramic composition is based on a solid solution system represented by x(NaNbO3)-y(BaNb2O6)-z(BiNb3O9). A problem with this solid solution system is that the mechanical quality factor (Qm) that significantly affects the driving performance of the piezoelectric oscillators, resonators and transducers is as low as about 100. Moreover, no description related to the direction of the spontaneous polarization is contained.
The present invention provides a compound having a tungsten bronze structure exhibiting a high Curie temperature, good insulating resistance and mechanical quality factor, and excellent piezoelectric properties.
The present invention also provides a novel compound having a lead-free tungsten bronze structure having a spontaneous polarization axis inclined with respect to the c axis of the crystal.